Conventionally, a known navigation device learns a new road, which is not registered in map data, based on a moving track of a movable body such as a vehicle. For example, as disclosed in JP-A-2005-172578, a road learning function enables to generate and store a new road data, which does not exist in map data. The road learning function enables to generate the new road data based on a running track from a point, at which the present position deviates from an existing road in the map data, to a point, at which the vehicle returns to an existing road in the map data. Whereby, the road learning function enables to learn the new road, which was not registered in the map data. Such a road learning function enables to perform a routing assistance using the new road without obtaining additional update information from an outside to update map data.
However, such a road learning function may learn various new roads, regardless of contribution to the routing assistance of the navigation device. For example, when an access road extends from an existing road around the destination of a routing assistance to the destination, and when the access road is learned as a new road, the leaned new road enables the vehicle to accurately arrive at the destination. In this case, the leaned new road is high in utility value. Thus, such a new road is conceived to contribute greatly to enhance accuracy of the routing assistance.
On the other hand, when the vehicle takes a side trip on the way to the destination, for example, a moving track of the vehicle in a parking lot may be learned as a new road. Such a new road is low in utility value when used for routing assistance to a destination. Thus, such a new road is conceived not to contribute to enhance accuracy of the routing assistance. Therefore, when such a new road, which is low in utility value, is learned as a learning object, effect of the road learning function may be impaired.